Nappers are typically large fabric processing machines that raise fuzz, termed pile or nap, on the surface of woven or knitted fabrics.
In most modern napper designs, unprocessed fabric enters the machine and passes over a series of rolls, known as worker rolls, that are themselves arranged around a cylinder. The worker rolls are driven to rotate either all against, or alternatively with and against, the direction of fabric progress. The cylinder is driven in relationship to the direction of fabric progress. In a double-acting mode in which the worker rolls are alternately driven with and against the direction of fabric progress, the cylinder rotates with the fabric. The cylinder rotates in the opposite direction when processing knit fabrics.
The worker roll surfaces are covered with sharp, densely packed hooked or straight wires, termed carding, which tug at the surface of the fabric. This process pulls and breaks the fabric fiber, creating the nap in the processed fabric.